1. Field of the Invention
This invention relates broadly in the field of electronic musical tone generators and in particular is concerned with the provision for a noise generator in a polyphonic tone synthesizer.
2. Description of the Prior Art
While the majority of musical tones are characterized by a well-defined pitch, there exists a large group of musical instruments which have no clearly defined pitch characteristic. Such instruments include the percussion family. For example, drums, cymbals, maracas, wood blocks, and tamborines. It is well known in the electronic music art that these "unpitched" musical sounds can be imitated using a random noise generator as the primary signal source. Examples of the application of random noise generators in an electronic musical instrument are contained in the U.S. Pat. No. 3,247,307 entitled "Electronic Musical Instrument."
The most common characteristic of noise generators is that they generate a noise signal which is called by the generic name of "white noise." White noise can be defined as a signal which is uniformly and randomly distributed in amplitude and has a power spectrum which is constant per unit bandwidth over the entire frequency region. No musical instrument, or in fact any real physical device, has a signal characteristic that approaches that of a white noise signal type. Instead, the musical instrument's spectrum tends to fall off at high frequencies in a manner similar to the response of a low-pass filter.
Noise signal sources having a spectrum that is not white are frequently called by the generic name of "pink noise." A more limited use of the term pink noise has been applied to a noise signal whose spectrum contains constant power per percentage bandwidth for all frequencies. The broader generic definition of pink noise will be used in the following.
Pink noise generators have been used in conjunction with the analog variety of tone generators, and it is evident that pink noise generators are also desirable adjuncts to digital musical tone generators. A method of generating an analog noise signal is described in the technical article; D. B. Keele, Jr., "The Design and Use of a Simple Pseudo Random Pink-Noise Generator," J. Audio Engineering Society, Vol. 21 (January/February 1973) pp. 33-41. The described system starts with a conventional shift register variety of a binary white noise generator. The output random sequence of "0" and "1" signal states are transformed by an analog filter to produce the desired pink noise signal.
It is obvious to those skilled in the art that the binary white noise generator output signals can be processed by a digital filter to produce a source of digital pink noise. One disadvantage to this conventional and straightforward approach is that a digital filter is not a simple and low clost implementation because it requires the use of one or more digital data multipliers.
The present invention provides a novel means for producing pink noise with adjustable spectral responses without using conventional digital filters in a digital tone generator of the type described in U.S. Pat. No. 4,085,644 entitled "Polyphonic Tone Synthesizer." It is a feature of the present invention that a flexible source of a pink noise signal is generatated without the use of either digital or analog filters.